Electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes (an anode and a cathode). An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. Liquid feed solid polymer fuel cells operate in a temperature range of from about 0° C. to the boiling point of the fuel, i.e., for methanol about 65° C., and are particularly preferred for portable applications. Solid polymer fuel cells include a membrane electrode assembly (“MEA”), which comprises a solid polymer electrolyte or proton-exchange membrane, sometimes abbreviated “PEM”, disposed between two electrode layers. Flow field plates for directing the reactants across one surface of each electrode are generally disposed on each side of the membrane electrode assembly. There is typically a backing layer (or gas diffusion layer) between the flow field plate and the MEA.
A broad range of reactants have been contemplated for use in solid polymer fuel cells, and such reactants may be delivered in gaseous or liquid streams. The oxidant stream may be substantially pure oxygen gas, but preferably a dilute oxygen stream such as found in air, is used. The fuel stream may be substantially pure hydrogen gas, or a liquid organic fuel mixture. A fuel cell operating with a liquid fuel stream wherein the fuel is reacted electrochemically at the anode (directly oxidized) is known as a direct liquid feed fuel cell.
A direct methanol fuel cell (“DMFC”) is one type of direct liquid feed fuel cell in which the fuel (liquid methanol) is directly oxidized at the anode. The following reactions occur:Anode: CH3OH+H2O→6H++CO2+6e−Cathode: 1.5O2+6H++6e−→3H2OThe hydrogen ions (H+) pass through the membrane and combine with oxygen and electrons on the cathode side producing water. Electrons (e−) cannot pass through the membrane, and therefore are collected and flow from the anode to the cathode through an external circuit driving an electric load that consumes the power generated by the cell. The products of the reactions at the anode and cathode are carbon dioxide (CO2) and water (H2O), respectively. The open circuit voltage from a single cell is about 0.7 to 1 volt. Several direct methanol fuel cells are connected in series to obtain greater voltage.
Other liquid fuels may be used in direct liquid fuel cells besides methanol—e.g., other simple alcohols, such as ethanol or ethylene glycol, or dimethoxymethane, trimethoxymethane, hydrazine and formic acid. Further, the oxidant may be provided in the form of an organic fluid having a high oxygen concentration—i.e., a hydrogen peroxide solution.
A direct methanol fuel cell may be operated on aqueous methanol vapor, but most commonly a liquid feed of a diluted aqueous methanol fuel mixture is used. It is important to maintain separation between the anode and the cathode to prevent fuel from directly contacting the cathode and oxidizing thereon (called “cross-over”). Cross-over results in a short circuit in the cell since the electrons resulting from the oxidation reaction do not follow the current path between the electrodes. To reduce the potential for cross-over of methanol fuel from the anode to the cathode side through the MEA, very dilute aqueous mixtures of methanol (for example, about 5 percent of methanol in water, wherein the methanol concentration is expressed in methanol weight as percent of the total weight of the aqueous mixture) are typically used as the fuel streams in liquid feed DMFCs.
The proton exchange membrane (PEM) is a solid, organic polymer, usually polyperfluorosulfonic acid, that comprises the inner core of the membrane electrode assembly (MEA). Commercially available polyperfluorosulfonic acids for use as PEM are sold by E.I. DuPont de Nemours & Company under the trademark NAFION®. Alternatively, composites of porous polymeric membranes impregnated with perfluoro ion exchange polymers, such as offered by W.L. Gore & Associates, Inc., can be used as the proton exchange membranes. The PEM must be hydrated to function properly as a proton (hydrogen ion) exchange membrane and as an electrolyte.
For efficient function of the fuel cell, the liquid fuel should be controllably metered or delivered to the anode side. The problem is particularly acute for fuel cells intended to be used in portable applications, such as in consumer electronics including cell phones, where the fuel cell orientation with respect to gravitational forces will vary. Traditional fuel tanks with an outlet at the bottom of a reservoir, and which rely on gravity feed, will cease to deliver fuel when the tank orientation changes. In addition, dipping tube delivery of a liquid fuel within a reservoir varies depending upon the orientation of the tube within the reservoir and the amount of fuel remaining in the reservoir. Accordingly, to facilitate use of liquid fuel cells in portable electronic devices, a liquid fuel reservoir that controllably holds and delivers fuel to a liquid fuel cell, regardless of orientation, is desired.
A liquid fuel reservoir containing a wicking member, such as a urethane foam, is disclosed in co-pending U.S. Non-Provisional patent application Ser. No. 10/183,943, filed Jun. 28, 2002.